What is the advantage of having a FCIP router in SRDF/A replication.

One of the main benefits is more flexibility in the number of RDF ports.  With the native Gige boards, you are installing in groups of 4, using FC ports, you can have whatever number is most appropriate for the workload.  The second benefit is a higher compression rate than with the native boards.  By offloading the compression and TCP encapsulation work to the network hardware, the FC port achieves a higher throughput than a Gige can.  Lastly, error recovery on less than optimal WAN circuits is handled far better by the network hardware than at the individual Gige port level.  The most solid SRDF replication design always use FCIP with RF fibre ports on a VMAX array. The cost difference usually is very small if any since a small Brocade 7800 isn't expensive and the FC IO modules are cheaper than the Gige modules. I would certainly look into the FCIP design first all the time.

you would use FCIP if your RAs were traditional FC ports.